new support for Raspberry Pi B+

[Dan Raymond]

Further testing with the 16GB Kingston reveals that it is unstable even 
at low speed.  I hit the following panic while executing "portsnap fetch 
update" on a r274416 build:

dev = mmcsd0s2a, ino = 155101, fs = /mnt/ufs
panic: ffs_freefile: freeing free inode
KDB: enter: panic
[ thread pid 8 tid 100056 ]
Stopped at      $d:     ldrb    r15, [r15, r15, ror r15]!
db>